Surface Oxidation of Supported Ni Particles and Its Impact on the Catalytic Performance during Dynamically Operated Methanation of CO2

Abstract

Abstract:The methanation of CO 2 within the power-to-gas concept was investigated under fluctuating reaction conditions to gather detailed insight into the structural dynamics of the catalyst. A 10 wt % Ni/Al 2 O 3 catalyst with uniform 3.7 nm metal particles and a dispersion of 21% suitable to investigate structural changes also in a surface-sensitive way was prepared and characterized in detail. Operando quick-scanning X-ray absorption spectroscopy (XAS/QEXAFS) studies were performed to analyze the influence of 30 s and 300 s H 2 interruptions during the methanation of CO 2 in the presence of O 2 impurities (technical CO 2 ). These conditions represent the fluctuating supply of H 2 from renewable energies for the decentralized methanation. Short-term H 2 interruptions led to oxidation of the most reactive low-coordinated metallic Ni sites, which could not be re-reduced fully during the subsequent methanation cycle and accordingly caused deactivation. Detailed evaluation of the extended X-ray absorption fine structure (EXAFS) spectra showed surface oxidation/reduction processes, whereas the core of the Ni particles remained reduced. The 300-s H 2 interruptions resulted in bulk oxidation already after the first cycle and a more pronounced deactivation. These results clearly show the importance and opportunities of investigating the structural dynamics of catalysts to identify their mechanism, especially in power-to-chemicals processes using renewable H 2 .

Highly Active Sm2O3‐Ni Xerogel Catalysts for CO2 Methanation

“…Coking would be possible; however, is very unlikely as the CO 2 methanation is not prone to coking and no methane could be observed when re‐reducing the catalyst . According to the literature, Ni hydroxide and oxide evolution is also unlikely to form at the applied reaction conditions, although, several studies point out the inhibiting effect of H 2 O by blocking adsorption sites ,. It is, however, known that Sm 2 O 3 tends to form carbonates which would also deactivate the catalyst reversibly and independently of the metal loading by blocking adsorption sites.…”

Dynamic Methanation of CO2 – Effect of Concentration Forcing

“…). This behavior was also observed by Mutz et al , when a periodic forcing between a CO 2 /H 2 and pure CO 2 atmosphere was applied on a Ni/Al 2 O 3 catalyst. The overshot in CO 2 concentration in Fig.…”

“…According to their study, a fraction of the Ni crystals is oxidized by O 2 impurities originating from CO 2 , which is observed with operando spectroscopy. Therefore, a constant decline of the methane content for a 30 s cycle duration between CO 2 /H 2 and pure CO 2 was observed in the study of Mutz et al . In our study, the methane signal is always the same through all 50 cycles for the long cycle times of 60 s (see Fig.…”

“…The periodic forcing can suffer from a fast oxidation of the nickel crystals in H 2 -lean atmosphere [17]. Mutz et al [15] reported a decrease in the activity of the catalyst for H 2 dropouts at 400°C during the methanation. According to their study, a fraction of the Ni crystals is oxidized by O 2 impurities originating from CO 2 , which is observed with operando spectroscopy.…”

Catalytic, Photocatalytic, and Electrocatalytic Processes for the Valorization of CO2

“…The methanation of CO 2 was studied under unsteady conditions to evidence the structural dynamics of the Ni/Al 2 O 3 catalyst [27]. Different operando characterization techniques were used, such as Quick-scanning X-ray Absorption Spectroscopy/Extended X-ray Absorption Fine Structure (XAS/QEXAFS) considering stops in the H 2 supply during the reaction while feeding oxidizing impurities to simulate the technical purity of CO 2 .…”